1. Field of the Invention
The present invention relates to a white LED (light emitting diode), and more specifically to a white LED with an improved light extracting section, and a method of manufacturing the LED.
2. Description of the Related Art
White LEDs that use sapphire (Al2O3) as a substrate have such a structure that an LED that emits UV light is formed on one of the main surfaces of the substrate and phosphor is arranged on the other main surface of the substrate. With this structure, UV light is converted by phosphor into white light. In the white LEDs of this type, a refractive index of sapphire, which serves as the substrate, is high as 1.76, and therefore the problem of loss of light due to reflection on a surface of substrate and on an interface arises. For this reason, only about 32% of light can be extracted between sapphire and air. Thus, it is difficult to efficiently extract light created inside the element to the outside.
In order to increase the light extracting efficiency of LED, there has been an attempt of increasing the transmittance by forming a regularly arranged structure (of recesses and projections) of a nanometer size on a surface of the light emitting element (Applied Physics Letters, 142, vol. 78, 2001, Jpn. J. Appl. Phys., L735, vol. 39, 2000). In this attempt, since the recesses and projections of the structure are arranged in nanometer size, the recess and projection region serves as such a layer that the refractive index smoothly changes from the surface of semiconductor to the air, for light. Therefore, the reflection does not occur, but light is completely passed through the layer.
On the other hand, in order to make a coarse surface, there is a conventionally known technique in which the surface is treated with hydrochloric acid, sulfuric acid, hydrogen peroxide or a mixture of these (Jpn. Pat. Appln. KOKAI Publication No. 2000-29949). According to this technique, recesses and projections of micron-sizes are formed by the surface treatment, and thus the multiple scattering on the recesses and projections is utilized to extract as much light as possible.
However, it is difficult to form excellently configured recesses and projections in a surface of sapphire by a technique described above. That is, sapphire is a very difficult material to etch, and therefore sapphire cannot be processed by a dry etching method using a resist mask sufficiently as desired. Even if the optimal etching gas is selected, the ratio in the etching rate with respect to the resist is about only 1 at best. For this reason, it is difficult to form a desired recess and projection configuration, especially, a recession and projection shape having a high aspect ratio of a size of a sub-micron order.
As described above, with the conventional technique, it is not possible to form an excellent recess and projection shape in a surface of a sapphire substrate, which has been a factor of lowering the light extracting efficiency in the white LED.